Various types of photosensors have been proposed and are known, including: those which employ inorganic materials such as CdS, CdSe and amorphous Si, those which have organic pigments dispersed in binder resins, and those which have organic pigments formed in layers by vacuum evaporation, spin coating or various other techniques.
Photosensors that employ inorganic materials have not been considered to be suitable for use over large areas of the meter order because the film forming methods that can be adopted for this purpose are limited. Photosensors that have organic pigments dispersed in binder resins are advantageous for use over large areas but commercially acceptable high optical response speeds have not yet been attained. Photosensors that have organic pigments formed in layers by vacuum evaporation, spin coating or various other known techniques have problems in connection with the control of molecular arrangement in the film or with its thickness.